1. Field of the Invention
The present invention relates to a signal processing method and a signal processing apparatus for performing image compression, for input image data, or expansion, for compressed image data, and a computer-readable medium and a data recording medium.
2. Description of the Related Art
Various systems have appeared that provide numerous services for the processing of digital image data. For example, available today are systems that provide services for saving and managing digital image data obtained while using scanners to scan negative film or while using digital cameras to take pictures, and systems that provide services for the transmission of such digital image data across networks.
FIG. 10 is a block diagram showing the transmission side of the related-art still picture transmission apparatus. The still picture transmission apparatus in FIG. 10 is the related-art example related to the transmittal of digital image data when a homepage is browsed or when a digital image data transmission service is provided, for example, for a video telephone, i.e., this is related to an improvement in an image data transmission technique employing the JPEG (Joint Photographic Experts Group) compression system, which is a generally known digital image data compression system.
First, a digital image creation unit 101 receives digital image data from an image pickup unit such as a CCD, or a recording medium such as a hard disk, and converts the digital image data into 8-bit data consisting of a luminance signal and a color difference signal. In order to perform the JPEG compression for the thus created digital image data, an image division/compression unit 102 divides one screen into a plurality of blocks, each of which is a well known square array of 8×8 pixels. As a result, one screen=blocks of n rows×m columns, and one block is a square array of 8×8 pixels. Then, DCT processing is performed for the 8×8 pixels in each of the thus obtained blocks. As is well known, for DCT processing, a matrix operation is performed for the original image data of 8×8 pixels by using a cosine coefficient transpose and a cosine coefficient, and a spacial coordinate axis is transformed into a frequency coordinate axis to divide the frequency into a low frequency element and a high frequency element.
Further, the image division/compression unit 102 creates two screens of n×m blocks, and thins blocks in the two block division screens A and B to obtain the checkerboard like patterns shown in FIGS. 11A and 11B. During the thinning process, when the screen A, for example, includes an odd number of rows and an odd number of block columns, normal quantization is performed, but when an odd number of rows and an even number of columns are included, only the DC coefficient is quantized and 0 is employed as the AC coefficient. In a case such as that for the division screen A in FIG. 11A, white blocks are regarded as normally quantized blocks, and shaded blocks are regarded as blocks of “0” and are thinned in a checkerboard like manner. It should be noted that in the well known quantization operation, the coefficient matrix of a square array obtained by the DCT processing is divided by using a square array quantization table, and while the low frequency element located at the upper left in the matrix is finely quantized, the high frequency element located at the lower right is only roughly quantized. In another quantization process to be performed in this case, only the DC coefficient located at the topmost left end is quantized, and all the other AC coefficients are regarded as 0. The thinning process using the two quantization operations is performed for the two screens A and B, and as a result, the division screens A and B shown in FIGS. 11A and 11B are obtained. Image data thinned using quantization are encoded by using Huffman coding, and the resultant data are output as JPEG compressed data by an image data transmission unit 103.
To receive the compressed image data output via a transmission network 104, such as the Internet, white blocks are extracted from the division screens A and B based on ID information and are synthesized to form the original screen, and the inverse transformation of JPEG compressed data is performed by an expansion unit, to decode the image data, and the resultant data are displayed (see JP-A-11-261824).
However, to perform image data compression or expansion at a higher speed and at a lower cost, when the data are still pixel data, effective means is required for reducing the volume of the data to be compressed, such as that required for still pictures, before the compression process is performed. However, were pixel data, to reduce their volume, simply thinned vertically or transversely, the resolution would be reduced vertically or transversely, and the image quality would be greatly affected. But when pixel data are thinned in a checkerboard like manner, a reduction in the vertical or the transverse resolution is prevented, and the image quality is less affected. On the other hand, since the pixel data are thinned in the checkerboard like manner, an operation for a square array, such as quantization using a quantization table, can not be performed. Therefore, both for a case wherein image data are compressed without the screen being divided into blocks and for a case, as described in JP-A-11-261824, wherein the screen is divided into blocks of 8×8 pixels, a pixel array can not be shaped like a square when the pixel thinning process is performed in advance. Thus, performing the compression process is impossible, and image data can not be compressed.
In order to resolve these shortcomings, one objective of the present invention is to provide a signal processing method and a signal processing apparatus, for enabling image data compression while reducing the amount of data to be compressed, without the image quality being deteriorated, and that can perform the compression and expansion of image data at a high speed and at a low cost, and a computer-readable medium and a data recording medium.